Electrodynamic loudspeakers usually comprise a cylindrical coil mechanically integral with an emissive acoustic surface also called diaphragm. The coil is generally borne by a straight mandrel integral with the diaphragm. This emissive acoustic surface is usually conical (cone) or spherical (dome) in shape. Loudspeakers generally have an axis of cylindrical symmetry, although elliptical loudspeakers exist. They also comprise a fixed magnetic circuit the function of which is to produce a radial magnetic field onto the coil, inside a gap.
To obtain a quality sound reproduction, it is desirable for the magnetic induction to be the most constant possible along a generating line of the gap, the one on which the coil is located and moves. Indeed, variations of this induction induce sound distortions when the coil moves.
Iron magnetic circuits according to the state of the art generally comprise an axially-magnetized annular or discoidal magnet and ferromagnetic parts intended for channelling magnetic flux through the coil. For example, patent application WO 96/04706, “Axially focused radial magnet voice coil actuator”, M. STRUGACH, proposes using radial magnets. Further, the proposed magnetic circuit comprises iron or soft ferromagnetic material.
Defects induced by the presence of iron in the magnetic circuit are now well known. Consequently, for a few years, ironless magnetic circuit structures have been proposed. For example, patent EP-0 503 860, “Transducer motor assembly”, W. HOUSE, does not explicitly mention the presence of iron and proposes using two axial magnets in mutual repulsion. The latter structure has been improved using a radial magnet between two axial magnets in patent EP-1 553 802, “Magnetic circuit and speaker”, OHASHI. In the latter document, the loudspeaker motor comprises a stack of three magnets having the same remanent magnetisation and alternate magnetic field polarizations, at 90° relative to each other, and the magnetic polarization orientations of which are such that the magnetic field loopback outside the magnets is done essentially on the gap side, as shown in FIG. 4 of this document.
Finally, magnetic circuits using triangular-section magnets have been proposed in patent application FR-05/53331, “Transducteur électrodynamique, applications aux haut-parleurs et géophones”, G. LEMARQUAND, V. LEMARQUAND and B. RICHOUX. If the latter magnetic circuits are efficient, they however need a costly machining of the magnets.